In general, a time of flight mass spectrometer has a flight space in which neither electric nor magnetic field is present. Into this space, ions that have been given an initial kinetic energy by an electric field are introduced, and the flight time of each ion is measured until it reaches an ion detector. Based on this flight time, various ion species are separated with respect to their mass-to-charge ratios. To improve the mass resolution of this type of mass spectrometer, it is preferable to make the flight distance of the ions as long as possible. However, if the flight space is straight, it is often difficult to linearly extend the flight distance due to the limited overall size of the apparatus and other factors. To address this problem, various structures for increasing the effective flight distance have been proposed.
For example, Japanese Unexamined Patent Application Publication No. 2005-78987 discloses a mass spectrometer having multiple electric fields arranged to form a closed-loop or spiral (pseudo-loop) orbit, such as a circular orbit or an “8”-shaped orbit. Ions are introduced into this orbit and fly along the orbit multiple times until they are separated according to their mass-to-charge ratios. Finally, the separated ions are detected by the ion detector. However, the flight time thereby measured has some errors resulting from various factors independent of the mass-to-charge ratio. Examples of such factors include: dispersion of the initial kinetic energy given to the ions; dispersion of the starting points of the ions; temporal change (or jitter) of each ion at the starting time; and temporal change (or jitter) of each ion at the point of detection by the ion detector. These errors will lower the analysis accuracy.
To solve this problem, the mass spectrometer disclosed in Japanese Unexamined Patent Application Publication No. 2005-79037 includes an ion detector capable of measuring the flight time (or elapsed time) of each turn of the ion in the loop orbit. Based on the detection signals of the ion detector, a flight time spectrum having a peak at each turn of the ion is created. Then, this spectrum data is Fourier-transformed to convert the time axis to a frequency axis. On the resultant frequency spectrum, each frequency peak corresponding to each mass-to-charge-ratio is identified to calculate the mass-to-charge ratio. In this data processing, the Fourier-transformation removes the aforementioned error factors that are independent of the mass-to-charge ratio. Therefore, the mass-to-charge ratio thereby calculated is very accurate. While the ions are flying along the loop orbit, even if one ion flying at a higher speed laps another ion flying at a lower speed due to the difference between their mass-to-charge ratios, the mass spectrometer can separately detect these ions having different mass-to-charge ratios. Thus, the measuring range of the mass-to-charge ratio is expanded.
The use of Fourier-transformation in a time of flight mass spectrometer having a loop orbit significantly enhances the accuracy and mass resolution of the analysis. However, to improve the accuracy, it is necessary to considerably increase the number of turns of each ion species. In practice, it may be 1000 turns or more, so that one cycle of analysis takes a long time. Therefore, this type of mass spectrometer is not suitable for a situation where the analysis needs to be repeated at short intervals of time. One example is an analysis using the mass spectrometer as a detector of a gas chromatograph or liquid chromatograph. In this case, the mass spectrometer needs to repeatedly analyze the sample eluting from the column of the chromatograph. If the time required for one cycle of analysis is long, the time resolution will be accordingly low and the detector may fail to detect some of the sample components.
In contrast, the normal type of time of flight mass spectrometer can repeat the analysis at much shorter intervals of time than the type having a loop orbit. Its time resolution can be set so high as to prevent the detection failure of the sample components. Moreover, as opposed to the Fourier-transformation type, the normal type directly receives (i.e. destroys) ions and has an accordingly high sensitivity, so that it can detect even a small amount of ions. However, the normal type is disadvantageous in that it is not high in analysis accuracy and mass resolution. In summary, in a mass spectrometric analysis of a sample eluting from a chromatograph, if it is necessary to avoid the detection failure of the sample components and also perform a high resolution analysis of some specific components, the same sample must be measured twice using different systems, which consumes time and labor.
To solve this problem, the present invention provides a mass spectrometer capable of appropriately switching its mass-analyzing operation between the first mode having high time resolution and the second mode having high mass resolution and high accuracy.